In the aesthetic restorative treatment in the dental field, dental crown restorative materials made of ceramics have hitherto been used clinically; however, most of such materials have been glass ceramics including leucite crystals (KAlSi2O6). The refractive index of the leucite crystals approximates to the refractive index of the surrounding glass matrix, accordingly the glass ceramics including the crystals have transparency, and consequently aesthetically excellent dental crown restorative materials have been able to be prepared. However, leucite crystals are dendrites, accordingly cannot suppress the development of cracks generated in the interiors of the glass ceramics, and hence the materials including leucite crystals have been unable to obtain high material strength.
Thus, recently, as glass ceramics to develop high strength, lithium silicate glass ceramics have been applied clinically. The lithium silicate glass ceramics are materials in which by heat treating lithium silicate glass compositions, characteristic forms of crystals (lithium disilicate and/or lithium metasilicate) are precipitated in high densities; and the lithium silicate glass ceramics have a structure in which these crystals are mutually entangled, and accordingly suppress the development of cracks and develop high material strength. At present, the use of these lithium silicate glass ceramics has been expanded to various applications in the dental field; examples of such a use include powdery porcelain materials for building up/burning and ceramic blank for press molding or CAD/CAM mechanical processing. Moreover, many conventional technologies have recently been reported on these lithium silicate glass compositions.
Patent Literature 1 describes the improvement of the material strength by allowing Al2O3 to be included in lithium silicate glass compositions, and thus intentionally also precipitating various crystals (such as lithium aluminum silicate compounds including spodumene) other than the main crystals (lithium disilicate and/or lithium metasilicate).
Patent Literature 2 describes the possibility of the improvement of the transparency, while the material strength is being maintained, by including ZnO in a content of less than 1.0% by weight in the lithium silicate glass composition, and by regulating the ratio between SiO2 and Li2O and the ratio between Al2O3 and K2O.
Patent Literature 3 describes the improvement of the material strength by including Al2O3 and additionally including ZrO2 and TiO2 in the lithium silicate glass composition, and thus intentionally also precipitating various crystals (such as lithium titanium oxide silicate and lithium aluminum silicate compounds including spodumene) other than the main crystals (lithium disilicate and/or lithium metasilicate).
In any of these conventional technologies, a glass composition includes Al2O3 in the lithium silicate glass composition. Al2O3 is a component functioning advantageously in the chemical durability, and accordingly a lithium silicate glass ceramic including Al2O3 develops a high chemical durability, for example, in such a way that the elution the glass components is suppressed when used in the oral cavity, and thus allows a stable glass composition to be constructed. In addition, Al2O3 is one type of glass forming oxide, thus reinforces the glass skeleton and contributes to the improvement of the durability of the glass, and also has an effect to suppress the devitrification (crystal precipitation) in the cooling process when glass blank is prepared.
However, when a lithium silicate glass composition includes Al2O3, in addition to the main crystals (lithium disilicate and/or lithium metasilicate) precipitated from the lithium silicate glass composition, various crystals (such as lithium aluminum silicate compounds including spodumene) are precipitated by heat treatment through the reaction with Li2O in the composition, accordingly the degradation of the material strength is caused, and consequently it is impossible to develop the high material strength demanded for dental glass ceramics. When P2O3 is added as a nucleating agent in the lithium silicate glass composition, Al2O3 and P2O3 in the composition react with each other by heat treatment and crystals such as aluminum phosphate are also precipitated, and hence the addition of P2O3 as a nucleating agent further promotes the degradation of the material strength. Moreover, in lithium silicate glass compositions of conventional technologies, the presence of Al2O3 produces the refractive index difference between such various types of crystals precipitated after heat treatment as described above and the residual glass phase, and consequently in the esthetics of the prepared dental crown restorative material, there has been a significant problem that, for example, lithium silicate glass ceramics after heat treatment become opaque.
In the preparation of the dental crown restorative material by a press method, having hitherto been performed, in which a glass or glass ceramic ingot is pressed into, a phosphate-based investment material is used as a mold. In this case, when Al2O3 is included in the composition of a lithium silicate glass or glass ceramic ingot composed of a lithium silicate glass composition, various crystals (surface reactive layer) such as aluminum phosphate are precipitated, at the time of pressing into, on the surface of the lithium silicate glass ceramics due to the reaction with the phosphate included in the investment material, and hence the degradation of the transparency or material strength is likely to be caused. The reactive layer on the surface of the lithium silicate glass ceramics leads to the surface roughness of the dental crown restorative material, and is a factor to degrade the technical work.
Accordingly, with the conventional lithium silicate glass compositions, it is impossible to efficiently precipitate the main crystals to achieve high strength, due to the effect of Al2O3 included in the composition, and it is impossible to develop the high material strength demanded for the dental glass ceramics because various crystals are precipitated due to the reaction with Li2O or P2O3 in the composition. The precipitation of various crystals leads to a result that the transparency as well as the material strength is adversely affected.
As described above, the ceramic dental crown restorative materials used for the aesthetic restorative treatment in the dental field are demanded to have the mechanical strength capable of withstanding the harsh occlusal pressure or the esthetics analogous to natural tooth; however, as affairs now stands, no lithium silicate glass compositions satisfy these required properties.